The goals of the proposed research are: (1) To measure in vivo in rats the permselectivity characteristics of the glomerular capillary membrane using H3-dextrans varying in molecular size from 18 to A44. Theoretical water and solute flux relations derived using nonequilibrium thermodynamics and macromolecular transport through small pores will be used in correlating experimental data. (2) The effects of selective variations in systemic hematocrit on the pre-glomerular, glomerular, and post-glomerular microcirculation in the rat will be investigated using isovolemic exchange transfusion. The systemic hematocrit is expected to be reduced to approximately 20 percent or elevated to approximately 70 percent under which conditions the determinants of glomerular ultrafiltration will be measured. (3) Electronic pattern recognition techniques and television microscopy will be used to measure erythrocyte velocities and capillary diameters in vivo in the rat cortical and papillary microcirculation. (4) The permselectivity properties of the rat renal glomerulus will be measured in animals in the early autologous phase of nephrotoxic serum nephritis. (5) Experiments in the rat renal papilla will be made to confirm that water is taken up and returned to the general circulation by the vasa recta. The transcapillary forces responsible for this uptake of water will be measured and interpreted by using appropriate mathematical models.